There is an increasing trend to use Stewart platforms to implement ultra-high precision tasks under large external loads (e.g. biomechanical testing) mainly due to their high stiffness, and high load carrying capacity. However, the backlash or joint clearance in the system can significantly degrade the accuracy. This work studied the application of actuation redundancy in a general Stewart platform to regulate the preloads on its active joints for the purpose of backlash prevention. A novel active preload control method was proposed to achieve a real-time approach that is robust to large six degree of freedom external loads. The proposed preload method applies an inverse-dynamics based online optimization algorithm to calculate the desired force trajectory of the redundant actuator, and uses a force control scheme to achieve the required force. Simulation results demonstrate that this method is able to eliminate backlash inaccuracies during application of large external loads and therefore ensure the precision of the system.